Can you mount GRX FC-RX600 110/80 chainrings on a Tiagra 4700 chainset?

The GRX chainset is 3.4mm (or possibly 2.5mm, depending on the source you read), outboard of the road chainset. The purpose of this is to allow wider tyres.

In addition, it's worth noting that road freehubs were traditionally 130mm wide, and MTB were 135mm wide. When discs were added to road bikes, the disc freehubs went to 135mm as well, but the non-disc freehubs stayed at 130mm.

At the same time that this happened, there were no new 'disc' chainsets released. They kept with the same chainline, which was 43.5mm for road double. This compared to 48.8mm for MTB double, which is the extra (5mm/2), plus another 2.8mm to cater for big fat tyres.

The chainline measurement at the front is from the middle of the BB to the middle of the chainrings (so around 2.5mm further out to the large ring, and 2.5mm further in to the big ring). The chainline at the back is the distance between the middle of the centreline of the bike, running from the middle of the bottom bracket to the rear hub.

The relevant measurement is then the chainstay length, which is the distance between these two points.

A compact road bike such as this one

https://www.giant-bicycles.com/int/tcr-advanced-1-disc-pro-compact

has 40.5mm chainstay length

whereas an endurance bike such as this one has 42.0mm chainstay length

https://www.giant-bicycles.com/int/defy-advanced-2-2021

Meanwhile an MTB such as this one is barely longer at 42.5mm

https://www.giant-bicycles.com/int/xtc-advanced-29-3-2021

which turns out to be the same as this GRX-equipped gravel bike

https://www.giant-bicycles.com/int/revolt-advanced-1-2021

It should be obvious that a shorter chainstay means you will get a bigger variation in chain angles when cross-chaining, so that with a particularly compact chainstay you could get rubbing when using the small/small combination.

That said, they still managed to release wider frames and wider disc hubs without having to make new road chainsets with chainline further out, and everyone was pretty much happy with their new disc road bikes.

Anyways, there was nothing particularly different about GRX as a groupset. You got:

• rear derailleurs that were compatible with road shifters but perhaps supported larger cassettes
• brifters which were largely the same as hydraulic road brifters, but with slightly different ergonomics
• identical brake calipers (except for the paint job) to those sold as 'road'
• the exact same bottom brackets used for 'road'

and finally you got this new chainset which had smaller chainrings because 'gravel'. Although Shimano in the past have used various funky chainring bolt spacing, all their current road double chainsets use 110mm BCD. This 110mm means that the smallest possible chainring is 33t, while there isn't really a largest one, but stiffness might dictate for a very large chainring you would use a bigger BCD.

Since the standard road chainset is now 50/34, to go any smaller (which makes sense, off-road), Shimano need to use a smaller BCD, which here is 80mm, for reasons that aren't clear, but probably for aesthetics and/or to make them incompatible with the millions of MTB chainrings around this size which are 96mm BCD or 104mm, and could have been fitted.

Note, btw, that the q-factor (width between the cranks) of the GRX chainset is only 5mm wider than the Ultegra/Dura-Ace/105 chainsets, and 1mm wider than 4700 chainset, whereas MTB chainsets tend to be around 30mm wider than road. This is of course not related to chainline but is influenced by the tyre width, in that big tyres will mean big q-factors.

If you look at this supposed 'GRX' bike:

https://masoncycles.cc/products/insearchof-grx then it boasts HUGE 27.5 x 2.8" or 29 x 2.4" tyres, but it uses a Deore XT 1x chainset, which obviously has massive q-factor and eliminates Shimano's incompatibility between its MTB front derailleurs and its road brifters.

The maximum tyre clearance of GRX 2x is officially 42mm, which is nominally 1.6"

If you look at a 'gravel' bike like this then it's using the a bog standard 50/34 road chainset

https://www.decathlon.co.uk/p/triban-rc-520-gravel-riding-bike/_/R-p-302303

and has 42.5mm chainstay length, and the bike works well with 38mm and possibly 40mm tyres

https://thesweetcyclists.com/widest-tire-decathlon-triban-rc-520/

(note that the 'RC 520 Gravel' and the 'RC 520' are the same frame and fork, it's just finishing kit that differs)

An apparently purpose-built-for-gravel frameset such as this one

https://www.decathlon.co.uk/p/men-s-gravel-bike-grvl-900-shimano-grx-titanium-frame/_/R-p-325632

has the same 42.5mm chainstay length and will go up to 45mm tyres, so a bit wider frame than a modern disc road bike that might also do for gravel.

Specailized fit the same frame with both standard road compact double chainset

https://www.specialized.com/us/en/diverge-base-e5/p/175291?color=290931-175291&searchText=96220-7344

and with a Praxis 1x road crankset

https://www.specialized.com/us/en/diverge-comp-e5/p/175289?color=290949-175289&searchText=96220-5349

and claim 47mm of tyre clearance, though supplied is 35 or 38mm. Given that this is accomplished with a 43.2mm chainstay length and a bog standard road chainline crankset then it's difficult to see that GRX brings much to the table.

It appears to me that with discs taking over, and manufacturers taking the opportunity to widen the resulting freehubs by 5mm, then road cranksets should probably have got 2.5mm wider anyway. But as I show above, this 2.5mm does not seem all that critical.

More critical tends to be induced incompatibility, e.g., the use of different pull ratios on MTB and road, so it is difficult to use road brifters with an MTB FD.

Note that front derailleur chainline specs are not exact numbers - the GRX chainset is specced at 46.9mm (I am not sure if this a misprint for 46.0mm), the FD is specced at 46.0mm, but plenty of people use road FDs (43.5mm) with the GRX chainset, and they work fine.

If you examine a GRX chainset

you can see that the driveside assembly is merely 'FC-RX600' - it is not FC-RX600-1 or FCRX600-10/-11, it's just FC-RX600, so the difference between the three models is the chainrings.

In addition, you can clearly see there the hole for the bolts smaller BCD inner chainring - which in this particular crankset has not been fitted. This is rather like a MTB triple chainset, which bolts the big/middle ring into one hole, and the small ring into a separate hole.

An alternative way of mounting a smaller chainring is like this:

With this the small chainring piggy backs onto the large one, and is not connected directly to the spider (in this case the large chainring is a direct mount (onto the spindle, rather than bolt on) type, though that doesn't really affect the connection to the small ring).

So your FC-4700 spider is designed to have two chainrings with the same BCD bolted to it, whereas a GRX spider has two separate sets of bolt holes for the different BCD chainrings.

I would further note that the bolt holes for the GRX inner ring are spaced inbound of the outer holes, whereas the spacing of road compact rings is achieved by placing the rings either side of the spider, such that the spacing is a function of the thickness of the spider, as well as of the hole thickness of the outer ring.

If you consider the chainline of the different rings:

1. GRX 1x 49.7mm quoted
2. GRX 2x 46.9mm quoted, which means approx 49.4mm outer and 44.4mm inner chainline, given typical 5mm spacing
3. Road 2x 43.5mm quoted, which means approximately 46.0mm for the outer and approx. 41mm for the inner

then it's clear that the GRX spider bolt holes must be physically outbound of the road spider bolt holes. So even if you could fit the GRX chainrings to the 4700 chainset, then it wouldn't really make it more 'gravelly', especially as we've noted above, nearly all the difference between 'gravel' and 'road' in terms of the drivetrain & brakes is 'marketing' (there is a difference in the frameset and fork in that for gravel these must be have at least around 35mm of clearance, but that is something that has come with many 'road' frames and forks with the movement to disc brakes, which eliminate the 'max clearance' that you had with caliper brakes), and you can make a very good 'gravel' bike with Tiagra 4720 hydraulic, with no real need for GRX at all.

Well ok, yes, the GRX does have slightly smaller chainrings but that's nothing 'Gravel' specific. The GRX chainring gap of 16t or 17 is designed so that they could reuse road front derailleurs, which are 16t gap. There are plenty of 46/30t chainsets, but they'll all use smaller BCD, by necessity - a Tiagra 4720 'gravel bike' with all Tiagra components except a 46/30t third-party chainset would not be much different from a GRX 2x10-speed bike.

Another popular option is to take a triple road crankset, remove the outer chainring (can be replaced with a chainguard), and then use a road front compact derailleur with inner rings at 16t spacing (e.g., 26t/42t). This gives 40mm/45mm/chainguard (50mm).

But any and all such smaller chainrings will REQUIRE a new chainset that supports a smaller BCD for sub 34t (or 33t) chainrings.